The present invention relates to a color space conversion device and a color space conversion method for converting RGB data of a color image into CMYK data, by which high quality color printing is realized.
These days, a wide variety of color printers are used as color image output devices for printing color images on paper. When such a color image output device outputs a color image, parameters called UCR (Under Color Removal) and BG (Black Generation) are set to appropriate or particular values, and the UCR/BG are used for outputting the color image.
For example, in an xe2x80x9cImage processing methodxe2x80x9d disclosed in Japanese Patent Application Laid-Open No.HEI2-9273 (Japanese Patent No.2728208), a line of RGB data is scanned in the horizontal scanning direction, and differential signals R(x)xe2x88x92G(x) and G(x))xe2x88x92B(x) (R(x): red signal, G(x): green signal, B(x): blue signal) are integrated on the line, and thereby an appropriate UCR (UCR characteristic line) for the line is determined depending on the two integrals.
In a xe2x80x9cPrinting devicexe2x80x9d disclosed in Japanese Patent Application Laid-Open No.SHO63-100895, when color space conversion is executed for achromatic color areas, the UCR/BG is corrected by use of a gamma table.
Generally, a color image input device such as a scanner, a digital camera, etc. handles and manages color images employing the RGB color space, while a color image output device such as a color printer manages color images employing the CMYK color space in many cases. A monitor (which operates as a color image input device when a color image displayed on the monitor is directly printed by a color printer) also manages color images employing the RGB color space. Therefore, color space conversion from RGB data into CMYK data becomes necessary when a color image inputted by a color image input device is outputted by a color image output device. The color space conversion from RGB data into CMYK data can be executed directly using a table called LUT (Logical Unit Table). However, in many cases, the color space conversion from RGB data into CMYK data is conducted by two steps: a first color space conversion from the RGB data into CMY data, and a second color space conversion from the CMY data into the CMYK data. The first color space conversion from RGB data into CMY data is usually executed by means of complementary color conversion, conversion using a color space conversion table, etc. The second color space conversion from CMY data into CMYK data is usually executed using the UCR/BG. The UCR/BG is usually defined as a value or a table, independently of the position of the CMY data in the CMY color space (independently of the position of the RGB data in the RGB color space).
However, in many cases, the optimum values of the UCR/BG change depending on the position of the CMY data in the CMY color space (depending on the position of the RGB data in the RGB color space). Concretely, when the CMY data is like an achromatic color (when the RGB data is like an achromatic color), it is desirable that most of the CMY data is replaced by xe2x80x9cKxe2x80x9d (black print), that is, the UCR/BG should be near 100%. On the other hand, when the chroma of the CMY data is high (when the chroma of the RGB data is high), it is desirable that the replacement by the xe2x80x9cKxe2x80x9d is reduced, that is, the UCR/BG should be smaller.
Therefore, when UCR/BG which have been optimized near the achromatic color area are employed, the xe2x80x9cKxe2x80x9d-print is used a lot with respect to CMY data in the achromatic color area, and thereby printed output like pure gray is obtained successfully with respect to the CMY data in the achromatic color area. However, with respect to CMY data far from the achromatic color area, the chroma of the CMY data is lowered undesirably due to the heavy use of the xe2x80x9cKxe2x80x9d-print. On the other hand, when UCR/BG which have been optimized near the achromatic color area are employed, the replacement by the xe2x80x9cKxe2x80x9d-print is limited with respect to CMY data having high chroma, and thereby printed output having high chroma is obtained successfully with respect to the CMY data having high chroma. However, with respect to CMY data in the achromatic color area, gray is necessitated to be expressed by the combination of C, M and Y since the replacement by the xe2x80x9cKxe2x80x9d-print is limited, and thus printed output like pure gray can not be obtained successfully.
In the conventional image processing method disclosed in Japanese Patent Application Laid-Open No.HEI2-9273, a plurality of UCRs are prepared, and an appropriate UCR to be used is selected from the UCRs based on the characteristics (the two line integrals) of the RGB data. However, the detection of the characteristics of the RGB data and the determination (selection) of the appropriate UCR based on the characteristics are executed with respect each line in the horizontal scanning direction. Therefore, even when the characteristics of the RGB data changed rapidly on the line (when achromatic areas and high chroma areas appeared alternately on the line, for example), the characteristics are integrated on the line and only one appropriate UCR is determined for the line based on the integrals. Therefore, the detection of the characteristics of the RGB data and the determination of the appropriate UCR can not be executed correctly by the conventional image processing method. Further, when the selection of an appropriate UCR from the prepared UCRs is executed with respect to each line, the selected UCR changes between lines, and thus color discontinuity occurs between the lines in the printed color image. However, no countermeasures for correcting the color discontinuity have been disclosed in the document.
The conventional printing device disclosed in Japanese Patent Application Laid-Open No.SHO63-100895 executes gamma correction to the UCR/BG when color space conversion is executed to achromatic color areas, in order to obtain printed output like pure gray for the achromatic color areas. However, the conventional printing device does not use the xe2x80x9cKxe2x80x9d-print a lot for expressing achromatic colors, and thus it is impossible for the conventional printing device to express pure gray in the printed output.
It is therefore the primary object of the present invention to provide a color space conversion device and a color space conversion method, by which correct detection of the characteristics of the RGB data (or the CMY data) and appropriate setting of the UCR/BG table (or the LUT) are executed with respect to each smaller unit of the RGB data so as to realize correct color space conversion from the RGB data to CMYK data, and thereby RGB data in the achromatic color area, for example, can be expressed as pure gray in the CMYK data and in the printed output.
Another object of the present invention is to provide a color space conversion device and a color space conversion method, by which the color discontinuity, which occurs due to the change of the UCR/BG table (or the LUT) which is used for color space conversion, can be corrected and eliminated.
In accordance with a first aspect of the present invention, there is provided a color space conversion device for executing color space conversion from RGB data to CMYK data, comprising a first color space conversion means, a CMY position detection means, two or more conversion tables, a conversion table synthesizer means, and a second color space conversion means. The first color space conversion means converts the RGB data into Cxe2x80x2Mxe2x80x2Yxe2x80x2 data. The CMY position detection means detects the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space with respect to each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data. The conversion tables are tables which have been optimized at particular positions in the CMY color space so as to be used for color space conversion from Cxe2x80x2Mxe2x80x2Yxe2x80x2 data to CMYK data. The conversion table synthesizer means synthesizes a suitable conversion table for each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data from the conversion tables, based on the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space which has been detected by the CMY position detection means. The second color space conversion means converts each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data into CMYK data by use of the suitable conversion table for the dot which has been synthesized by the conversion table synthesizer means.
In accordance with a second aspect of the present invention, in the first aspect, the conversion tables are UCR/BG (Under Color Removal/Black Generation) tables.
In accordance with a third aspect of the present invention, in the first aspect, the CMY position detection means calculates:
k=min (Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)
a=max (Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)xe2x88x92min (Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)
as the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space.
In accordance with a fourth aspect of the present invention, in the first aspect, the color space conversion device further comprises a continuation table means. The continuation table means outputs one or more continuation parameters based on the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space which has been detected by the CMY position detection means. The continuation parameters are used by the conversion table synthesizer means for synthesizing the suitable conversion table correcting and eliminating color discontinuity due to change of the conversion table which is used by the second color space conversion means for the color space conversion from Cxe2x80x2Mxe2x80x2Yxe2x80x2 data to CMYK data.
In accordance with a fifth aspect of the present invention, in the first aspect, the first color space conversion means converts the RGB data into the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data by means of complementary color conversion.
In accordance with a sixth aspect of the present invention, in the first aspect, the first color space conversion means converts the RGB data into the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data by means of conversion using a LUT (Logical Unit Table).
In accordance with a seventh aspect of the present invention, there is provided a color space conversion device for executing color space conversion from RGB data to CMYK data, comprising an RGB position detection means, two or more conversion tables, a conversion table synthesizer means, and a color space conversion means. The RGB position detection means detects the position of the RGB data in the RGB color space with respect to each dot of the RGB data. The two or more conversion tables are tables which have been optimized at particular positions in the RGB color space so as to be used for color space conversion from RGB data to CMYK data. The conversion table synthesizer means synthesizes a suitable conversion table for each dot of the RGB data from the conversion tables, based on the position of the RGB data in the RGB color space which has been detected by the RGB position detection means. The color space conversion means converts each dot of the RGB data into CMYK data by use of the suitable conversion table for the dot which has been synthesized by the conversion table synthesizer means.
In accordance with an eighth aspect of the present invention, in the seventh aspect, the conversion tables are LUTs (Logical Unit Tables).
In accordance with a ninth aspect of the present invention, in the seventh aspect, the RGB position detection means calculates:
a=max(R,G,B)xe2x88x92min(R,G,B)
as the position of the RGB data in the RGB color space.
In accordance with a tenth aspect of the present invention, in the seventh aspect, the color space conversion device further comprises a continuation table means. The continuation table means outputs one or more continuation parameters based on the position of the RGB data in the RGB color space which has been detected by the RGB position detection means. The continuation parameters are used by the conversion table synthesizer means for synthesizing the suitable conversion table correcting and eliminating color discontinuity due to change of the conversion table which is used by the color space conversion means for the color space conversion from RGB data to CMYK data.
In accordance with an eleventh aspect of the present invention, there is provided a color space conversion method for executing color space conversion from RGB data to CMYK data, comprising a first color space conversion step, a CMY position detection step, a conversion table synthesis step, and a second color space conversion step. In the first color space conversion step, the RGB data is converted into Cxe2x80x2Mxe2x80x2Yxe2x80x2 data. In the CMY position detection step, the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space is detected with respect to each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data. In the conversion table synthesis step, a suitable conversion table is synthesized for each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data from preliminarily prepared conversion tables. The conversion tables are tables which have been optimized at particular positions in the CMY color space so as to be used for color space conversion from Cxe2x80x2Mxe2x80x2Yxe2x80x2 data to CMYK data. The synthesis of the suitable conversion table is executed based on the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space which has been detected in the CMY position detection step. In the second color space conversion step, each dot of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data is converted into CMYK data by use of the suitable conversion table for the dot which has been synthesized in the conversion table synthesis step.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, the conversion tables are UCR/BG (Under Color Removal/Black Generation) tables.
In accordance with a thirteenth aspect of the present invention, in the CMY position detection step of the eleventh aspect, values:
k=min(Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)
a=max(Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)xe2x88x92min(Cxe2x80x2, Mxe2x80x2, Yxe2x80x2)
are calculated as the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space.
In accordance with a fourteenth aspect of the present invention, in the eleventh aspect, the color space conversion method further comprises a continuation parameter generation step. In the continuation parameter generation step, one or more continuation parameters are generated based on the position of the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data in the CMY color space which has been detected in the CMY position detection step. The continuation parameters are used in the conversion table synthesis step for synthesizing the suitable conversion table correcting and eliminating color discontinuity due to change of the conversion table which is used in the second color space conversion step for the color space conversion from Cxe2x80x2Mxe2x80x2Yxe2x80x2 data to CMYK data.
In accordance with a fifteenth aspect of the present invention, in the first color space conversion step of the eleventh aspect, the RGB data is converted into the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data by means of complementary color conversion.
In accordance with a sixteenth aspect of the present invention, in the first color space conversion step of the eleventh aspect, the RGB data is converted into the Cxe2x80x2Mxe2x80x2Yxe2x80x2 data by means of conversion using a LUT (Logical Unit Table).
In accordance with an seventeenth aspect of the present invention, there is provided a color space conversion method for executing color space conversion from RGB data to CMYK data, comprising an RGB position detection step, a conversion table synthesis step, and a color space conversion step. In the RGB position detection step, the position of the RGB data in the RGB color space is detected with respect to each dot of the RGB data. In the conversion table synthesis step, a suitable conversion table is synthesized for each dot of the RGB data from preliminarily prepared conversion tables. The conversion tables are tables which have been optimized at particular positions in the RGB color space so as to be used for color space conversion from RGB data to CMYK data. The synthesis of the suitable conversion table is executed based on the position of the RGB data in the RGB color space which has been detected in the RGB position detection step. In the color space conversion step, each dot of the RGB data is converted into CMYK data by use of the suitable conversion table for the dot which has been synthesized in the conversion table synthesis step.
In accordance with an eighteenth aspect of the present invention, in the seventeenth aspect, the conversion tables are LUTs (Logical Unit Tables).
In accordance with a nineteenth aspect of the present invention, in the RGB position detection step of the seventeenth aspect, a value:
a=max(R,G,B)xe2x88x92min(R,G,B)
is calculated as the position of the RGB data in the RGB color space.
In accordance with a twentieth aspect of the present invention, in the seventeenth aspect, the color space conversion method further comprises a continuation parameter generation step. In the continuation parameter generation step, one or more continuation parameters are generated based on the position of the RGB data in the RGB color space which has been detected in the RGB position detection step. The continuation parameters are used in the conversion table synthesis step for synthesizing the suitable conversion table correcting and eliminating color discontinuity due to change of the conversion table which is used in the color space conversion step for the color space conversion from RGB data to CMYK data.
In accordance with twenty-first through thirtieth aspects of the present invention, there are provided computer-readable record mediums storing programs for instructing a computer or a DSP (Digital Signal Processor) to execute the color space conversion methods of the eleventh through twentieth aspects of the present invention.